Polyesters such as poly(ethylene terephthalate) (PET) are widely used in the form of molded articles, bottles, containers, food trays, fibers, sheeting, and film. These PET polyesters are generally processed at high temperatures such as 500.degree. F.-570.degree. F. (260.degree. C.-300.degree. C.) and undergo severe hydrolytic degradation unless they are carefully dried prior to processing. Such drying operations are time consuming and expensive.
The effects of hydrolytic degradation in copolyesters such as these generally may be a loss in molecular weight, loss in inherent viscosity (I.V.), and/or lack of uniformity in products made therefrom, reduction in mechanical properties such as bubbles in extruded sheets, and the like.
U.S. Pat. No. 2,996,477 assigned to Eastman Kodak Company describes the use of 0.01 to 10.0 weight % N,N'-diphenyl-p-phenylenediamine or N,N'di-2-naphthyl-p-phenyldiamine in poly(cyclohexanedimethanol terephthalate) polyesters to provide improved thermal stability when molded parts are stored at elevated temperatures. This patent does not mention the minimization or elimination of drying the polyester prior to melt extrusion.
In the Journal of Applied Polymer Science, Vol. 16, pp. 3253-3263, 3257 (1972), entitled "Thermal and Hydrolytic Degradation of Poly(1,4-cyclohexane-dimethylene terephthalate), describes the thermal and hydrolytic degradation of PCT homopolyester at moisture levels of 0.012 weight %, 0.027 weight %, and 0.55 weight % at extrusion temperatures of 300.degree. C., 310.degree. C. and 320.degree. C. with residence times of 3.5, 7.0 and 17.5 minutes. This article teaches that residence time, temperature, moisture content of the supply polymer, and their interaction all contribute significantly to the degradation of the polymer. It teaches specifically on page 3261 that "As shown for moisture levels above 0.027% in the supply polymer, the experimental values for I.V. breakdown are attributable to the moisture content of the supply polymer."
In Polymer Engineering and Science, November, Vol. 12, No. 6, (1972), entitled "Kinetics of the Thermal Degradation of Poly(1,4-cyclohexylenedimethylene terephthalate", Gregory and Watson teach the kinetics of thermal degradation of 1,4-cyclohexylenedimethylene terephthalate at 295.degree.-330.degree. C. in a small, prototype melt extruder. This article teaches drying of all materials for 8 hours at 110.degree. C. under vacuum then purged for 16 hours at room temperature with a mixture of dry nitrogen and dry air to thoroughly dry the samples prior to processing.
U.S. Pat. No. 5,169,499 describes paper machine felts made from a copolymer comprising terephthalic acid, 1,4-methylolcyclohexane and isophthalic acid. There is no mention of thermal or hydrolytic stability of polyesters during melt processing. There is no mention of minimization or elimination of the drying step prior to melt processing.